1. Field of the Invention
The present invention relates to an optical information transmission system, and more particularly to an optical pick-up device for recording information to and reproducing information from an optical information storage medium.
2. Description of Prior Art
An optical pick-up device is an apparatus for recording information on an optical disc or reading out recorded information from an optical disc by using a laser light beam generated in an optical disc drive system. Generally, the optical pick-up device includes: a light source for producing the laser light beam; a light beam splitter for directing the laser light beam emitted from the light source toward an objective lens, which focuses the laser light beam onto a recording surface of an optical disc; and a photo-detector for receiving the light beam reflected by the recording surface, the reflected light beam containing information read from the recording surface.
In order to accurately record and reproduce data, it is desirable for an optical pick-up device to minimize loss of light beams passing therethrough. This improves the efficiency of utilization of the light beams, and reduces the size of light beam focusing dots impinging on the recording surface. A small dot size reduces noise and enhances the signal to noise ratio.
Generally, it is difficult to eliminate errors of recording information on an optical disc or reading out the recorded information from the optical disc due to aberration caused by spherical lenses employed in a convention optical pick-up device. Accordingly, aspheric lenses are employed in optical pick-up devices for partially eliminating the aberration. On the other hand, incident light beams can also cause aberration. Even though linear polarized light beams are employed as the incident light beams, the linear polarized light beams can be split into a P-polarized component and an S-polarized component, which can also cause aberration.
In order to overcome the above-described problems, improved optical pick-up devices such as that shown in FIG. 5 have been developed. The optical pick-up device 100 comprises a light source 110, a linear polarized light beam converter 120, a polarized light beam splitter 130, a splitter 140, an aspheric lens 150, and an optical disc 160. A light beam emitted from the light source 110, such as a semiconductor laser, passes through the linear polarized light beam converter 120 and is converted into a linear polarized light beam with a P-polarized component and an S-polarized component. Then the linear polarized light beam with the P-polarized component and the S-polarized component enters the linear polarized light beam splitter 130. Any P-polarized component of the linear polarized light beam is reflected away by the linear polarized light beam splitter 130, whereas the linear polarized light beam with the S-polarized component passes through the linear polarized light beam splitter 130 unimpeded. Then at the splitter 140, part of the linear S-polarized light beam passes through the splitter 140 unaltered, and a remaining part of the linear S-polarized light beam is reflected away by a reflecting face 142 of the splitter 140. The unaltered part of the linear S-polarized light beam is then focused onto a recording surface (not labeled) of the optical disc 160 by the aspheric lens 150. The recording surface reflects the light beam such that the reflected light beam contains recorded information read from the recording surface. The reflected light beam passes back through the aspheric lens 150, is reflected by the reflecting face 142 of the splitter 140, and is received by a photo-detector (not shown).
The optical pick-up device 100 can partially eliminate the problem of aberration. However, the linear polarized light beam with the P-polarized component is completely reflected by the linear polarized light beam splitter 130, so that almost half of the linear polarized light beam is wasted. The efficiency of utilization of light beams in the optical pick-up device 100 is correspondingly low.